The Long Term Evolution (LTE) communication standard uses an Orthogonal Frequency Division Multiple Access (OFDMA) protocol. In an OFDMA system, each user equipment (UE) is assigned a frequency sub-channel and a time slot in a physical channel for its communications with a base station, access point, or evolved NodeB (eNodeB). It is important in an OFDMA system to maintain both time and frequency synchronization. If frequency synchronization is lost then orthogonality between the various sub-carriers assigned to other UEs is also lost, which results in interference between UEs. If time error is present, system performance will be degraded due to received signal constellation rotation. Therefore, it is required in LTE that each UE maintains time and frequency synchronization with an eNodeB to which the UE is connected. In particular, all uplink signaling from UEs must be received at the eNodeB base station at the right time, within the boundaries of the cyclic prefix window (about five microseconds for LTE). This is particularly critical for multi-user multiple-input multiple-output (MU-MIMO) implementations of LTE.
To maintain uplink time synchronization, the eNodeB first needs to measure the delay of uplink signaling from each UE. Specifically, the eNodeB can measure a timing error or delay, from each UE, such as when the UE moves relative to its distance from the eNodeB, which will use up some of the UE's delay spread immunity designed into the cyclic prefix window. The actual timing measurement can be taken on an uplink reference signal channel, such as the Physical Uplink Shared Channel (PUSCH). Upon detecting an uplink timing error, the eNodeB can then send to each UE a correction message with a desired Time Advance.
A problem arises in that the PUSCH channel is unique per each UE, and is not always scheduled to carry data, and even when it does, the timing error measurement may be too noisy and unreliable. A regular low pass filter solution can remove noise. However, for high speed traffic such as video streaming, the PUSCH can be scheduled as often as every one millisecond sub-frame, but for low speed traffic such as PING, it may be active only once per one-thousand milliseconds. Therefore, PUSCH measurements can be unreliable, and using a regular low pass filter solution will fail to support both low and high speed data services. In addition, there can be residual synchronization errors in the physical channel after a Media Access Control (MAC) layer synchronization effort. This requires further timing compensation in the eNodeB. Moreover, although timing and frequency error estimate methods based on Cyclic Prefix (CP) correlation are well-known for OFDM signals, these techniques can not be applied to OFDMA systems, especially to an eNodeB receiver where multiple users have their own timing and frequency errors that cannot be separated from each other in PUSCH.
Accordingly, what is needed is a technique to correct the uplink timing error of multiple UEs. It would be of further benefit if this could be accomplished in a noisy environment. It would also be of further benefit if this could be accomplished using uplink timing measurements of unreliable PUSCH signals.
Skilled artisans will appreciate that common but well-understood elements that are useful or necessary in a commercially feasible embodiment are typically not depicted or described in order to facilitate a less obstructed view of these various embodiments of the present invention.